Top emission organic light emitting diode display using auxiliary electrode to prevent voltage drop of upper electrode and method of fabricating the same

ABSTRACT

An organic light emitting diode (OLED) display. The OLED display includes: a lower electrode formed on a layer on an insulating substrate having a thin film transistor. The lower electrode is electrically connected to the thin film transistor. An auxiliary electrode is formed on the same layer as the lower electrode, and a pixel defining layer is formed on edges of the lower electrode, thereby defining an opening which exposes a portion of the lower electrode. An organic layer is formed on the portion of the lower electrode exposed by the opening, and an upper electrode is formed on an entire surface of the insulating substrate and electrically connected to the auxiliary electrode. An edge of the auxiliary electrode may have a taper angle of at least 90°.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 11/101,176, filed Apr. 6, 2005, which claims priority to andthe benefit of Korean Patent Application No. 10-2004-0023900, filed Apr.7, 2004, and Korean Patent Application No. 10-2004-0024016, filed Apr.8, 2004, the entire contents of both of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic light-emitting diode (OLED)display and a method of fabricating the same. More particularly, thepresent invention relates to a top emission OLED display using anauxiliary electrode for preventing or reducing a voltage drop of anupper electrode such that the top emission OLED display may belarge-sized and a method of fabricating the same.

2. Description of the Related Art

A conventional top emission active matrix organic light-emitting diode(AMOLED) display uses a transparent cathode electrode in order to emitlight toward a sealing substrate.

In general, a transparent conductive material such as indium tin oxide(ITO) or indium zinc oxide (IZO) is mainly used as the transparentcathode electrode. However, in order to function as the cathodeelectrode, a metal material having a low work function is thinlydeposited on one side of the transparent conductive layer that comes incontact with an organic layer to form a semitransparent metal layer, andthen the ITO or IZO is thickly deposited on the semitransparent metallayer.

In the above process, since the ITO or IZO layer is formed after theorganic layer is formed, the ITO or IZO layer should be formed by a lowtemperature deposition method in order to minimize damage of the organiclayer due to heat or plasma. When the low temperature deposition methodis used, however, the quality of the ITO or IZO layer is deterioratedand its specific resistance is increased.

When the specific resistance of the cathode electrode is increased,voltage differences are generated between near regions and far regionsfrom a portion where a power source is input due to a voltage dropdepending on positions of the pixels rather than uniformly applying acathode voltage to all pixels. As a result, non-uniformity of brightnessand image characteristics may be generated and power consumption mayincrease.

Also, due to the voltage drop, it is difficult to apply to amiddle-sized or large-sized top emission AMOLED display.

In order to solve the above problem, Shoji Terada et al. introduced amethod of forming an auxiliary electrode for preventing a voltage dropof an upper electrode on a pixel-defining layer 285 in “54.5L: Late-NewsPaper: A 24-inch AM-OLED Display with XGA Resolution by Novel SeamlessTiling Technology,” SID Symposium Digest 34, 1463 (2003).

A conventional top emission OLED display will now be described withreference to the attached drawings.

FIG. 1 is a partial cross-sectional view of a conventional top emissionOLED display, showing only a portion corresponding to a thin filmtransistor, a pixel electrode, and a capacitor.

Referring to FIG. 1, a buffer layer 110 is formed on an insulatingsubstrate 100. An active layer 120 including source and drain regions121 and 125 is formed on the buffer layer 110. A gate electrode 141 anda lower electrode 147 of a capacitor are formed on a gate-insulatinglayer 130. Formed on an interlayer insulating layer 150 are source anddrain electrodes 161 and 165 connected to the source and drain regions121 and 125 through contact holes 151 and 155, respectively, and anupper electrode 167 of the capacitor connected to one of the source anddrain electrodes 161 and 165, for example, the source electrode 161.

A passivation layer 170 is formed on the entire surface of theinsulating substrate 100. A lower electrode 180, i.e., a pixelelectrode, is formed on the passivation layer 170 as an anode electrodeof an electroluminescent (EL) device connected to one of the source anddrain electrodes 161 and 165, for example, the drain electrode 165,through a via hole 175. A pixel defining layer 185 having an opening 189which exposes a portion of the lower electrode 180 is formed. An organiclayer 190 is formed on the lower electrode 180 in the opening 189. Then,an auxiliary electrode 193 for preventing a voltage drop of an upperelectrode is formed on the pixel defining layer 185, and an upperelectrode 195 serving as a cathode electrode is formed on the entiresurface of the insulating substrate 100.

However, according to the above method, in a process of forming theauxiliary electrode 193, when a semitransparent metal layer used as theauxiliary electrode 193 is deposited and patterned on the pixel defininglayer 185, the organic layer 190 may be damaged. Also, a masking processshould be added to form the auxiliary electrode 193, which leads to thecomplication of the process.

SUMMARY OF THE INVENTION

The present invention, therefore, solves aforementioned problemsassociated with conventional displays by providing an organic lightemitting diode (OLED) display using an auxiliary electrode to prevent orreduce a voltage drop of an upper electrode.

In exemplary embodiments of the present invention, a top emission OLEDdisplay capable of preventing or reducing a voltage drop of an upperelectrode improves brightness and image characteristics such that thetop emission OLED display can be large-sized.

According to an exemplary embodiment of the present invention, an OLEDdisplay includes: a lower electrode formed on a layer on an insulatingsubstrate having a thin film transistor, the lower electrode beingelectrically connected to the thin film transistor; an auxiliaryelectrode formed on the same layer as the lower electrode; a pixeldefining layer formed on edges of the lower electrode, thereby definingan opening which exposes a portion of the lower electrode; an organiclayer formed on the portion of the lower electrode exposed by theopening; and an upper electrode formed on an entire surface of theinsulating substrate and electrically connected to the auxiliaryelectrode.

An edge of the auxiliary electrode may have a taper angle of at least90°. The edge of the auxiliary electrode may have a taper angle between90° and 135°. The auxiliary electrode may prevent or reduce a voltagedrop of the upper electrode.

The upper electrode may be electrically connected to the auxiliaryelectrode through at least one side of the auxiliary electrode.

The lower electrode and the auxiliary electrode may be made of aconductive material having a work function larger than that of amaterial of the upper electrode.

The lower electrode and the auxiliary electrode may be made of amaterial having low specific resistance and high reflectivity.

The lower electrode and the auxiliary electrode may be made of a singlelayer or a multilayer.

The lower electrode and the auxiliary electrode may be made of Al, Mo,MoW, Ti, Ag/ITO, Ag/Mow, or MoW/Al(Nd)/ITO.

The lower electrode and the auxiliary electrode may be thicker than theorganic layer and may each have a thickness of at least 3,000 Å.

The auxiliary electrode may be arranged in a linear pattern.

The auxiliary electrode may be arranged in a grid pattern.

The auxiliary electrode may be connected to a cathode inlet terminal ofa pad portion.

A top surface of the auxiliary electrode may have a length which isgreater than or equal to that of a bottom surface of the auxiliaryelectrode.

The length of the top surface of the auxiliary electrode may have arange from a value equal to the length of the bottom surface of theauxiliary electrode to a sum of twice the thickness of the auxiliaryelectrode and the length of the bottom surface of the auxiliaryelectrode.

According to another exemplary embodiment of the present invention, amethod of fabricating an OLED display includes: concurrently forming anauxiliary electrode and a lower electrode on an insulating substrateincluding a thin film transistor, the lower electrode being electricallyconnected to the thin film transistor; forming a pixel defining layer onedges of the lower electrode, thereby defining an opening which exposesa portion of the lower electrode; forming an organic layer on theportion of the lower electrode exposed by the opening; and forming anupper electrode on an entire surface of the insulating substrate, theupper electrode being electrically connected to the auxiliary electrode.

According to yet another exemplary embodiment of the present invention,an OLED display includes: a lower electrode formed on an insulatingsubstrate having a thin film transistor, the lower electrode beingelectrically connected to the thin film transistor; an auxiliaryelectrode formed on the insulating substrate, the auxiliary electrodehaving an edge with a taper angle greater than 90°; a pixel defininglayer formed on edges of the lower electrode, thereby defining anopening which exposes a portion of the lower electrode; an organic layerformed on the portion of the lower electrode exposed by the opening; andan upper electrode formed on an entire surface of the insulatingsubstrate and electrically connected to the auxiliary electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will be describedin reference to certain exemplary embodiments thereof with reference tothe attached drawings in which:

FIG. 1 is a partial cross-sectional view illustrating a conventional topemission organic light emitting diode (OLED) display;

FIG. 2 is a partial cross-sectional view illustrating a top emissionOLED display according to a first exemplary embodiment of the presentinvention;

FIGS. 3A to 3C are partial cross-sectional views illustrating a processof forming a top emission OLED display according to the first exemplaryembodiment of the present invention;

FIG. 4 is a partial cross-sectional view illustrating an auxiliaryelectrode for preventing or reducing a voltage drop of an upperelectrode according to an exemplary embodiment of the present invention;and

FIGS. 5A to 5D are partial plan views that illustrate an OLED displaywith an auxiliary electrode according to an exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which certain exemplaryembodiments of the invention are shown. This invention may, however, beembodied in different forms and should not be construed as limited tothe embodiments set forth herein. Like reference numerals refer to likeelements throughout the specification. Also, when a layer is describedas being formed on a substrate in this specification, the layer may beformed directly on the substrate, or it may be formed on the substratewith one or more other layers that are formed therebetween.

FIG. 2 is a partial cross-sectional view that illustrates a top emissionorganic light emitting diode (OLED) display according to a firstexemplary embodiment of the present invention, partially showing R, G,and B unit pixels of the top emission OLED display.

Referring to FIG. 2, an active matrix organic light emitting diode(AMOLED) display according to the first exemplary embodiment of thepresent invention includes a lower electrode 220 electrically connectedto source and drain electrodes 211 and 215 of a thin film transistor onan insulating substrate 200 per each R, G, and B pixel, and an auxiliaryelectrode 223 for preventing or reducing a voltage drop of an upperelectrode. The auxiliary electrodes 223 are formed between the lowerelectrodes 220 of the R, G, and B pixels and on the same layer as thelower electrodes 220.

Also, the AMOLED display includes a pixel defining layer 230 formed onlyon an edge of the lower electrode 220 other than the auxiliary electrode223 to be separated by each R, G, and B pixel and to form an opening 235that exposes a portion of the lower electrode 220.

Also, the AMOLED display includes an organic layer 240 formed on thelower electrode 220 exposed by the opening 235 and the pixel defininglayer 230. The organic layer 240 is not formed on at least one side ofthe auxiliary electrode 223.

Also, the AMOLED display includes an upper electrode 250 formed on theentire surface of the insulating substrate 200 and electricallyconnected to at least one side of the auxiliary electrode 223.

FIGS. 3A to 3C are partial cross-sectional views which illustrate aprocess of forming a top emission OLED display according to an exemplaryembodiment of the present invention, partially showing a capacitor C, athin film transistor, and an electroluminescent (EL) device connected tothe thin film transistor.

Referring to FIG. 3A, an active layer 320 including source and drainregions 321 and 325 per each R, G, and B pixel is formed on aninsulating substrate 300, on which a buffer layer 310 is formed.

After forming the active layer 320, a gate insulating layer 330 isformed on the insulating substrate 300 (i.e., on the buffer layer 310),and then a conductive material is deposited and patterned to form a gateelectrode 341 and a lower electrode 347 of the capacitor.

Then, an interlayer insulating layer 350 is formed, and contact holes351 and 355 that expose portions of the source and drain regions 321 and325, respectively, are formed.

After forming the contact holes 351 and 355, a conductive material suchas MoW is deposited and patterned to form source and drain electrodes361 and 365 that are electrically connected to the source and drainregions 321 and 325 through the contact holes 351 and 355, respectively,and an upper electrode 367 of the capacitor connected to one of thesource electrode 361 and the drain electrode 365, for example, thesource electrode 361, thereby forming the thin film transistor and thecapacitor.

After forming the source and drain electrodes 361 and 365 and the upperelectrode 367 of the capacitor, a passivation layer 370 is formed on theentire surface of the insulating substrate 300 (i.e., on the interlayerinsulating layer 350), and a via-hole 375 that exposes one of the sourceand drain electrodes 361 and 365, for example, the drain electrode 365,is formed.

A lower electrode 380, i.e., an island-shaped pixel electrode, which iselectrically connected to the drain electrode 365 through the via hole375, is formed and, at the same time, an auxiliary electrode 383 forpreventing or reducing a voltage drop of the upper electrode to beformed hereafter is formed between the lower electrodes 380 of R, G, andB pixels.

At this time, the lower electrode 380 and the auxiliary electrode 383should be provided with at least one edge having a taper angle of 90° ormore. That is, at least one edge should be reverse tapered such that theorganic layer may be separated by the auxiliary electrode 383 whileforming the organic layer hereafter.

The auxiliary electrode 383 and the lower electrode 380 should be madeof a conductive material having a work function larger than that of anupper electrode material to be formed in the following process. By wayof example, the auxiliary electrode 383 and the lower electrode 380should be made of Al, Mo, MoW, Ti, Ag/ITO, Ag/Mow, MoW/Al(Nd)/ITO, or amaterial that may be used as a reflecting layer or an anode electrodehaving low specific resistance in order to reduce or minimize a voltagedrop of a cathode electrode and high reflectivity in order to increasereflectivity of an organic layer to be formed in the following process.Also, the lower electrode 380 and the auxiliary electrode 383 may bemade of a single layer or a multilayer. Also, the lower electrode 380and the auxiliary electrode 383 should be formed to be sufficientlythicker than the organic layer to be formed hereafter. By way ofexample, the auxiliary electrode 383 may be formed to have a thicknessof 3,000 Å or more.

Referring to FIG. 3B, a pixel defining layer 385 with an opening 389that exposes a portion of the lower electrode 380 is formed. At thistime, the pixel defining layer 385 should not be formed on the auxiliaryelectrode 383, and should be formed only on an edge of the lowerelectrode 380.

After forming the pixel defining layer 385, an organic layer 390 isformed on the opening 389. In other words, the organic layer 390 isformed on the portion of the lower electrode exposed by the opening 389.The organic layer 390 may be composed of a plurality of layers dependingupon functions thereof. In general, the organic layer 390 has amulti-layered structure including a light-emitting layer, and at leastone among a hole injection layer (HIL), a hole transport layer (HTL), ahole blocking layer (HBL), an electron transport layer (ETL) and anelectron injection layer (EIL).

At this time, the organic layer 390 is formed on the entire surface ofthe insulating substrate 300 (with other layers therebetween) and is notformed on at least one side of the auxiliary electrode 383. This isbecause the pixel defining layer 385 is formed at a predetermined acuteangle such that the organic layer 390 may be deposited on the pixeldefining layer 385, however, the auxiliary electrode 383 has a reverselytapered structure in which the taper angle of at least one edge is 90°or more and is formed to be sufficiently thicker than the organic layer390.

Referring to FIG. 3C, an upper electrode 395 that serves as a cathodeelectrode is formed on the entire surface of the insulating substrate300 (with other layers therebetween). At this time, the upper electrode395 has a double-layered structure of a semitransparent metal layer anda transparent conductive layer, wherein the semitransparent metal layeris formed by thinly depositing a metal material having a low workfunction and the transparent conductive layer is formed by thicklydepositing a transparent conductive material such as ITO and IZO.

Since the organic layer 390 is not formed on at least one side of theauxiliary electrode 383, the upper electrode 395 is electricallyconnected to at least one side of the auxiliary electrode 383. Thus,since the upper electrode 395 is electrically connected to at least oneside of the auxiliary electrode 383, it is possible to prevent or reducethe voltage drop of the upper electrode 395.

FIG. 4 is a partial cross-sectional view of an OLED, partiallyillustrating only an auxiliary electrode for preventing or reducing avoltage drop of an upper electrode according to an exemplary embodimentof the present invention.

Referring to FIG. 4, an auxiliary electrode 410 for preventing orreducing a voltage drop of an upper electrode according to an exemplaryembodiment of the present invention should be provided with at least oneedge having a taper angle α of 90° or more. By way of example, the taperangle may be 90° to 135°. That is, at least one edge has a reverselytapered structure.

This is because, when the taper angle α of the edge of the auxiliaryelectrode 410 is less than 90°, the organic layer is not cut on the sideof the auxiliary electrode 410 and may go over the auxiliary electrode410 while forming the organic layer.

Also, when the taper angle α of the edge of the auxiliary electrode 410is 135° or more, the length b of the bottom surface of the auxiliaryelectrode 410 is much shorter than the length a of the top surfacethereof so that the auxiliary electrode 410 may collapse. Also, when theOLED is driven, stress and strong electric field is generated above theedges of the auxiliary electrode 410.

In the case the auxiliary electrode 410 has a trapezoidal section, whenthe length of the top surface of the auxiliary electrode 410 is a, thelength of the bottom surface thereof is b, the thickness of theauxiliary electrode 410 is h, and an angle obtained by subtracting 90°from an alternate angle of the taper angle α of the edge of theauxiliary electrode 410 is α′, the following relationship is establishedbetween the length a of the top surface of the auxiliary electrode 410and the length b of the bottom surface thereof.b+h tan α′≦a≦b+2h tan α′(0°≦α′≦45°)  Equation 1

Since α′ is 0° to 45°, the length a of the top surface of the auxiliaryelectrode 410 should be b≦a≦b+2h.

That is, the length a of the top surface of the auxiliary electrode 410should be greater than or equal to the length b of the bottom surfacethereof and less than or equal to the sum of twice the thickness h ofthe auxiliary electrode 410 and the length b of the bottom surfacethereof.

FIGS. 5A to 5D are partial plan views that illustrate an OLED displaywith an auxiliary electrode for preventing or reducing a voltage drop ofan upper electrode according to an exemplary embodiment of the presentinvention.

Referring to FIG. 5A, an auxiliary electrode 520 for preventing orreducing a voltage drop of an upper electrode according to an exemplaryembodiment of the present invention is connected to an inlet terminal510 of a pad portion of an insulating substrate 500.

Referring to FIG. 5B, the auxiliary electrode 520 has a grid form, and alower electrode 530, i.e., a pixel electrode, is formed in an islandshape in each grid of the auxiliary electrode 520. When the auxiliaryelectrode 520 has the grid form, it is also possible to reduce a voltagedrop of a cathode electrode.

It can be seen in FIG. 5B that a via-hole 540 is formed on each lowerelectrode 530.

Referring to FIG. 5C, the island-shaped lower electrodes 530 arearranged as a matrix of rows and columns and an auxiliary electrode 520′is arranged as lines between the adjacent lower electrodes 530 arrangedin a column direction.

Referring to FIG. 5D, an auxiliary electrode 520″ is arranged in a rowdirection between the adjacent island-shaped lower electrodes 530arranged as a matrix of rows and columns.

As described above, according to the exemplary embodiment of the presentinvention, since the auxiliary electrode for preventing or reducing thevoltage drop of the upper electrode and the lower electrode areconcurrently formed, the auxiliary electrode for preventing or reducingthe voltage drop of the upper electrode may be formed to prevent orreduce a voltage drop of the cathode electrode without an additionalmask process. Also, it is possible to manufacture a middle-sized orlarge-sized OLED display by preventing or reducing the voltage drop ofthe cathode electrode.

As described above, according to the present invention, the auxiliaryelectrode for preventing or reducing the voltage drop of the upperelectrode may be formed to prevent or reduce the voltage drop of thecathode electrode and to thus provide an OLED display that is capable ofpreventing or reducing non-uniformity of brightness and/or imagecharacteristics.

Also, since the auxiliary electrode for preventing or reducing thevoltage drop of the upper electrode and the lower electrode areconcurrently formed, a cathode bus line may be formed without anadditional mask process.

Also, it is possible to provide an OLED display with low powerconsumption by preventing or reducing the voltage drop of the cathodeelectrode.

Although the present invention has been described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that a variety of modifications and variations may bemade to the present invention without departing from the spirit or scopeof the present invention defined in the appended claims, and theirequivalents.

1. A method of fabricating an organic light emitting diode display, themethod comprising: concurrently forming an auxiliary electrode and alower electrode on an insulating substrate including a thin filmtransistor, the lower electrode being electrically connected to the thinfilm transistor; forming a pixel defining layer on edges of the lowerelectrode, thereby defining an opening which exposes a portion of thelower electrode; forming an organic layer on the portion of the lowerelectrode exposed by the opening and on substantially an entire top sideof the auxiliary electrode, the organic layer contacting substantiallythe entire top side of the auxiliary electrode from a first edge of thetop side to a second edge of the top side opposite the first edge; andforming an upper electrode on an entire surface of the insulatingsubstrate, the upper electrode being electrically connected to theauxiliary electrode.
 2. The method as claimed in claim 1, wherein theauxiliary electrode prevents or reduces a voltage drop of the upperelectrode and is electrically connected to the upper electrode through alateral side of the auxiliary electrode.
 3. The method as claimed inclaim 1, wherein the lower electrode and the auxiliary electrode areformed thicker than the organic layer.
 4. The method as claimed in claim3, wherein the lower electrode and the auxiliary electrode are eachformed having a thickness of at least 3,000 Å.
 5. The method as claimedin claim 1, wherein the upper electrode is electrically connected to theauxiliary electrode through at least a lateral side of the auxiliaryelectrode.